1. Field of the Invention
This invention generally relates to an apparatus for controlling a throttle actuator which drives a throttle valve provided in a suction pipe of an internal combustion engine installed in an automobile or the like and, more particularly, to an apparatus that utilizes a brushless motor for the throttle actuator.
2. Description of the Prior Art
As disclosed in Japanese Patent Public Disclosure (Kokai) No. 35040/1987, it has previously been the practice to utilize a stepping motor as the motor of such throttle actuators. FIG. 1 shows such a conventional apparatus which comprises an accelerator pedal 21, an accelerator pedal sensor 22 for sensing the degree of depression of the pedal 21 and a throttle valve control circuit 23 including an analog-to-digital (A/D) converter 23A, a central processing unit (CPU) 23B, and a latch 23C. The apparatus further includes a motor driver 24 adapted to receive a drive control rate signal from the latch 23C, a stepping motor 25 driven by the motor driver 24 to control the degree of opening of a throttle valve 26, a return spring 26a for the throttle valve 26, and a throttle opening sensor 27 for sensing the degree of opening of the throttle valve 26, the sensor 27 being connected to the A/D converter 23A.
The operation of this conventional apparatus will be described below. An output signal from the accelerator pedal sensor 22 which is of a magnitude corresponding to the degree of depression of the accelerator pedal 21 is read by the throttle valve control circuit 23 which, in turn, produces a driving control signal in response thereto and feeds it to the motor driver 24 to drive the stepping motor 25. Thus, the motor driver 24 drives the stepping motor 25 in accordance with the driving control signal to adjust the opening of the throttle valve 26. The degree of opening of the throttle valve 26 is detected by the throttle opening sensor 27 and fed back to the throttle valve control circuit 23 to determine whether or not a predetermined degree of opening has been established.
It is advisable to substitute a brushless motor for the above-mentioned stepping motor 25, because the latter usually has a relatively low operating speed, a relatively high degree of vibration, or a relatively low level of motor efficiency. The brushless motor is operated in such a manner that the magnetic poles of the rotor are detected by means of an electronic circuit, instead of using the brushes of a direct current motor, and changing the current to the stator windings in accordance with the detected signal. Japanese Patent Public Disclosure (Kokai) No. 206248/1987 discloses a method of eliminating the circuit for detecting the position of the magnetic poles, but such a method is useless when the rotor is not rotated, that is, upon starting.
A conventional apparatus for controlling a throttle actuator uses an electronic circuit to detect the magnetic poles of the rotor of the brushless motor and therefore involves drawbacks in that, if any fault occurs in the magnetic pole position detection circuit such as to produce an abnormal magnetic pole position detection signal, the brushless motor may stop, which would result in the control of the throttle valve operation not being performed.